Chemical and mechanical properties

Compared to coals, raw biomasses have higher moisture and volatile content and lower char and ash content, leading to lower energy density and heating value. The low heating value translates directly to a need to burn many times larger amount of raw biomass to obtain the same amount of energy as received from one unit of coal. (La Nauze, 1987;

Vassilev et al., 2010) Figure 3 presents carbon and hydrogen content in relation to heating value for different fuels. The approaches to improve biomass properties are discussed in chapter 2.1.3. The chemical properties also make biomass more reactive than coal (Vassilev et al., 2010), and some biomasses are known to decay and even spontaneously combust while in storage, due to biological and thermochemical reactions (Van Loo &

Koppejan, 2010). The high moisture content may also make the fuel freeze in cold storage (Mattsson, 1990).

Figure 3. Comparison of volatile, carbon and hydrogen contents of fuels with respect to the higher heating value. Figure after Eija Alakangas (2005) with permission.

The chemical composition is a fundamental aspect of any fuel, and it is strongly linked with the reaction characteristics and possible reaction products. The chemical composition of biomass depends on for example the type, place of harvest and part of plant utilized. Sulfur, nitrogen, alkali, chlorine, polycyclic aromatic hydrocarbons, and heavy metal content of the biomass are of special interest as they can form compounds harmful to the boiler or the environment. Another issue is the ash behavior, as a low ash

melting temperature is not desirable in combustion applications. (Khan et al., 2009;

Vassilev et al., 2010)

The mechanical properties affect the storage and transport of the biomass, and also the fuel feeding system of the combustor (La Nauze, 1987). The main mechanical properties are particle size, shape and density, while other properties are not often considered if the biomass is not pelletized to a fixed size where it is wished to remain. Compared to coal, biomass is softer as a bulk, which is largely due to loose packing, but also due to lower strength of the material. The main challenge with biomass is typically maintaining the flowability, which is strongly affected by the irregular particle size and shape and the high moisture content (mechanical and liquid bridging) along with the angle of repose as well as internal and external friction factors. (Mattsson, 1990; Wu et al., 2011)

Miao et al. (2011) report that the energy consumption of grinding increased with the decreasing particle size. The effect of moisture content did not affect the comminution energy requirements with large particles, while having a significant impact with smaller particles. Specific comminution energy consumption could be significant compared to the fuel heating value when producing fine powders. (Miao et al., 2011) While the breaking of the largest fuel particles might be desirable, the generation of very fine fuel dust is not, as it poses health issues and increased difficulties in transportation and fuel feeding (Dai et al., 2012; Van Loo & Koppejan, 2010).

2.1.2.1 Particle size and shape distributions

For utilization in energy production, the fuel has to be processed into a suitable size range for the application. This means grinding or milling to achieve suitable particle size distribution (PSD) for the fuel feeding and specific combustion method. The particle size and shape distributions depend on the type of biomass and the (pre)treatment, such as cutting, shaving, or grinding, for example. To minimize fuel processing costs, the fuel is ground down (if necessary) only to a particle size which is manageable by the fuel transport and feeding systems (Van Loo & Koppejan, 2010). Due to these reasons, the PSD of biomass can be very wide, from very fine (order of µm) to very coarse particles (order of 10 cm) (Publication I ref. data).

The determination of particle shape is not a simple task, unless dealing with regularly shaped particles (such as spheres, tetrahedrons, cylinders or hexahedrons) which can be expressed with a few different dimension parameters. A classification of particle shape by Mandø & Rosendahl (2010) is presented in Table 1. It has been shown that mineral material, such as coal and sand (Figure 4) has a rather spherical, though irregular shape with a narrow distribution, with average roundness or circularity around 0.6. (Publication I ref. data; Ulusoy & Igathinathane, 2014). For biomass, several authors, have reported wide distributions of irregular and non-spherical shape (Cui & Grace, 2007; Doroodchi et al., 2013; Guo et al., 2012; Guo et al., 2014; Mattsson, 1990; Miao et al., 2011).

Table 1. Classification of particle shape (Mandø & Rosendahl, 2010).

Spherical Non-spherical

Regular Polygons,

low aspect ratio spheroids

Cubes, cylinders, disks, tetrahedrons, high aspect ratio spheroids Irregular Pulverized coal, sand,

many powders, particulate matter

Pulverized biomass, flakes, splinters, agglomerates

For irregular particles, several dimensions can be measured, and ultimately these particles require 3D measurement to determine their shape, surface area and volume accurately.

Several 2D and 3D shape factors have been presented in the literature (and summarized in Publication I) to reduce the shape to a single number, which typically describes how much the particle deviates from a circle or a sphere of the same surface area or volume.

Recently full 3D laser scanning of irregular particles has been presented by Bagheri et al.

(2015) to obtain the true particle shape. The method took 2 hours per particle to obtain the shape information (Bagheri et al., 2015).

Rosendahl et al. (2007) have presented an example of milling straw and the resulting particle size and shape distributions. Guo et al. (2012) have studied the effect of grinding to shape of biomass, and they found that the particles retained their elongated, stick-like shape throughout the grinding process. The particle elongation was reduced as the particle size reduced. This result was related to plant growth direction and the anisotropic cell bond strength, resulting in an elongated shape. Similarly, coal particles break along grain boundaries, leading to retention of a roughly similar spherical shape. (Guo et al., 2012) Mattsson (1990) points out that irregularly shaped biomass particles can be “hooked”, heavily curved particles, which tend to interlock with other particles, causing interlocking and bridging. Zulfiqar et al. (2006) have studied the co-firing of coal with sawdust and woodchips, and report the “physical form” of the biomass (shape and size) to be a major contributor to the flowability of the coal-biomass-mixture. Mattsson & Kofman (2002) state that particle shape is the most important factor in fuel flowability, as the long, thin and hooked particles are more prone of bridging and blocking. Guo et al. (2014) report findings on the angle of repose of blends of spherical material and biomass (images of particles illustrated in Figure 4), stating that differences in surface roughness, size and shape contribute to the flowability of the mixture. The measured angles of repose increased linearly as a function of the biomass share in the mixture. While the smaller biomass particle typically increased flowability, the sawdust surface roughness was attributed to decreasing the flowability in the mixture, despite the fine particle size. (Guo et al., 2014)

Figure 4. Scanning electron microscope images of granular material. (a) Glass beads. (b) PVC.

(c) Al2O3. (d) Quartz sand. (e) String. (f) Coal. (g) Rice straw. (h) Sawdust (Guo et al., 2014).

2.1.2.2 Density

The irregular shape of biomass particles and their ability to absorb water make the measurement of particle or material density challenging. Biomass density is often reported as bulk density, which is affected by the particle packing (shape and size distributions), moisture content and external pressure applied. (Miao et al., 2011) Therefore, while more difficult to measure, material density should be utilized. Examples of material densities are presented in Table 2 for biomasses and coal. Compared to coal, biomasses have typically lower density, which is partially explained by a low ash content.

The moisture content also increases the biomass density, dry biomass being lighter than wet biomass. The results presented by Miao et al. (2011) indicate that both the material and bulk densities are affected by the particle size, the effect being larger for bulk density.

Table 2. Examples of reported material densities from Green & Perry (1997).

Material Birch, yellow Fir, Douglas Pine, Norway Spruce, white Pine charcoal Average material

density [kg/m³] 705 510 545 450 370

Material Oak charcoal Peat Limestone Anthracite (coal) Lignite (coal) Average material

density [kg/m³] 530 370 2450 1550 1250